Solid compressor blades with mid-span dampers providing stability have been the standard throughout the gas turbine industry. It is desirable to gain aerodynamic efficiencies by removing the mid-span damper. This leads to greatly increased chord width to gain equivalent stiffness in the shroudless blade as well as the necessity for hollowness to reduce weight.
In the background description of U.S. Pat. No. 3,628,226 reference is made to fabrication of hollow blades by milling forged blanks. These blanks are twisted and thereafter bonded together. The bonding surfaces are accordingly not in a single plane which makes it difficult to apply the compressive bonding load.
The improvement described in U.S. Pat. No. 3,628,226 comprises first grooving flat blanks, and forming the grooved blank into a preliminary airfoil surface without longitudinal curvature. Next the blanks are machined flat on the inner surface. These blanks are then diffusion bonded to form a rough blade, and thereafter the blade is twisted to form the final blade.
In order to produce the desired final blade without complex final machining, the starting form must be established. This must be established with knowledge of a predictable and constant metal flow during the forming operation. With the prior art forming techniques this has not been found to be possible. Necking of material thickness on stretching, wrinkling of compressed portions, die friction, and hot or cold spots in the dies have been some of the problems.